With the development of data processing, the research and development of materials having a high non-linear optical effect are under way for the purpose of realizing optical logical element, optical switch, etc. as a basic technique for optical computer. As non-linear optical materials there have heretofore been known an inorganic ferroelectric material such as LiNbO.sub.3, BaTiO.sub.3 and KH.sub.2 PO.sub.4, a quantum well semiconductor made of GaAs or the like, an organic single crystal such as 4'-nitrobenzylidene-3-acetamino-4-methoxyaniline (MNBA) and 2-methyl-4-nitroaniline (MNA), a conjugated organic high molecular compound such as polydiacetylene and polyarylene vinylene, and semiconductor particle-dispersed glass having Cds, CdSSe, etc. dispersed in glass.
Extensive studies have been made of semiconductor particle-dispersed glass as a favorable non-linear optical material having a high non-linear optical susceptibility and a high response in combination since Jain and Lind found in 1983 that a so-called color glass filter having finely divided semiconductor particles dispersed in glass exerts a high tertiary non-linear optical effect (as disclosed in J. Opt. Soc. Am., 73, 647 (1983)).
The preparation of this type of glass is normally accomplished by a so-called melt quenching process which comprises heat-melting a mixture of glass or its starting material as a dispersant and a metal or semiconductor powder to prepare a molten glass, casting the molten glass onto a metal plate or the like so that it is rapidly cooled to the vicinity of room temperature to obtain a supercooled glass solid solution having elements constituting a semiconductor dissolved therein as ions, and then subjecting the solid solution to heat treatment at a proper temperature for a predetermined period of time to cause the precipitation of a particulate semiconductor.
However, this melt quenching process requires heating of a semiconductor material to a temperature as high as not lower than 1,000.degree. C., causing the decomposition and evaporation of the semiconductor material. Thus, the kind of the semiconductor to which this melt quenching process can be applied and the amount of the semiconductor which can be added are limited, giving an obstacle to the realization of a material having a higher non-linear optical effect for practical use.
As another process there has been proposed a process which comprises sputtering a polycrystalline single semiconductor such as CdS and CdTe onto glass or SiO.sub.2 as a target to prepare a semiconductor particle-dispersed glass (as disclosed in J. Appl. Phys., 63 (3), 957 (1988), JP-A-2-307832 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), etc.).
As a further process there has been proposed an evaporation or gas phase process which comprises dispersing a particulate semiconductor in a high molecular compound as a matrix other than glass (as disclosed in JP-A-3-119326, JP-A-3-140035, etc.).
These gas phase processes make it possible to dope the matrix with a semiconductor in a larger amount than in the foregoing melt quenching process. However, these gas phase processes require an expensive apparatus regardless of whether the matrix used is inorganic or organic. Further, since these gas phase processes can form a film only at a low speed, it is difficult to form a thick film although they can be used to form a thin film. Moreover, since the form of the element thus obtained is limited to thin film, its use is also limited.
As an approach for overcoming these problems there has been proposed a process which comprises allowing a particulate semiconductor or metal to be dispersed and held in a silica gel matrix produced by sol-gel method so that a semiconductor particle-dispersed glass can be prepared at a low temperature.
As such an approach there has been known a method which comprises dispersing a particulate semiconductor previously prepared by CVD method or the like in a solution of a hydrolyzation product of silicon alkoxide (sol), and then gelating the sol so that the particulate semiconductor is solidified in glass (JP-A-2-271933), a method which comprises adding a particulate semiconductor to a sol containing a silane coupling agent or allowing the particulate semiconductor to be precipitated in the sol, and then gelating the sol so that the particulate semiconductor is solidified in glass (JP-A-3-199137), a method which comprises forming a silica gel containing cadmium acetate, and then reacting the cadmium acetate with hydrogen sulfide gas to cause a particulate cadmium sulfide to be precipitated in the silica gel to obtain a semiconductor particle-dispersed glass [transactions of 1989 annual conference of the Ceramic Society of Japan, Session No. 2F20, J. Non-Cryst. Solids, 122, 101 (1990)], etc.
However, if a tetraalkoxysilane commonly used in the prior art sol-gel method is used, the material is subject to cracking at the step of drying the gel. Further, if a thin film is formed on a substrate to prepare an optical element, a sufficient thickness cannot be provided. Accordingly, in order to obtain an element having a sufficient thickness, an approach is employed which comprises applying the material to a substrate to a thickness of about 0.1 .mu.m, calcining the thin film at a temperature of not lower than hundreds of degrees centigrade, applying the material to the film to a small thickness, and then repeating this procedure until a proper thickness is obtained.
If as the method for dispersing a particulate semiceductor in a silica gel matrix formed by sol-gel method there is used a method which comprises dispersing a particulate semiconductor which has previously been prepared by a separate method in a sol, a step of preparing a particulate semiconductor is needed, complicating the procedure. Further, a particulate semiconductor having a particle diameter of hundreds of nanometers used is remarkably difficult to handle, giving undesirable problems in the production process. Such a particulate material can be easily condensed and thus can be hardly dispersed uniformly in a medium.
JP-A-2-271933 discloses that ultrasonic dispersion or the addition of a surface active agent provides an effective improvement in the dispersion of a particulate material. However, the use of ultrasonic dispersion is disadvantageous in that the condensation of a particulate material is unavoidable during the application and drying when a thin film is formed. Further, the addition of a surface active agent is disadvantageous in that the surface active agent thus added decomposes or volatilizes away during heat treatment, causing the particulate material to be recondensed.
As a countermeasure against the foregoing problem, JP-A-3-199137 proposes that a silane coupling agent be used instead of surface active agent in an attempt to solve the problem of condensation of particulate material. This proposal features that the silane coupling agent acts like a surface active in a sol and is bonded to a matrix upon hydrolyzation. Thus, the silane coupling agent becomes thermally stable and cannot be hardly decomposed. The above cited patent also proposes as an approach for solving the handling problem of particulate semiconductor incorporated in a sol a method which comprises adding a semiconductor material in the form of solution, and then applying a paired ion source solution or reactive gas to the material to produce a particulate semiconductor in a sol. However, this method is disadvantageous in that the particulate material thus precipitated in the solution has difficulties in dispersion, making it difficult to uniformly precipitate a microcrystalline semiconductor having a quantum sizing effect. Thus, the effect of this method leaves something to be desired.
The method which comprises preparing a gel solid containing semiconductor material ions, and then subjecting the gel solid to post-treatment with hydrogen sulfide gas or the like to cause a particulate semiconductor to be precipitated has no problems of complicated regulations on handling of particulate material or no problems of ununiform dispersion but is disadvantageous in that a very toxic gas such as hydrogen sulfide must be used, endangering the workers and hence requiring a complicated procedure for safety. This method comprising a post-treatment for precipitation of a particulate material is advantageous in that it comprises uniformly dissolving in a solution a material soluble in the reaction medium as a material of particulate semiconductor or metal, making it possible to uniformly precipitate the particulate material in the medium but is disadvantageous in that there is no proper solvent depending on the materials used, restricting the concentration of the particulate material which can be added.
The sol-gel method can be effected at a lower temperature than the melt quenching process but requires heating to a temperature as high as about 600.degree. C. Thus, in order to solve the problems of heat decomposition of semiconductor material, a process which can be operated at an even lower temperature has been desired.
On the other hand, various studies have been made also of the particulate semiconductor or metal which exerts a non-linear optical effect. In particular, studies have been made-of the use of a cuprous halide which is expected to exert a high tertiary non-linear optical effect because it produces excitons having a small Bohr diameter and thus exerts a good effect of confining excitons (Journal of Non-Crystalline Solids, 134 (1991), pp. 71-76, Journal of American Ceramic Society, 74 (1991), pp. 238-240, Journal of the Chemical Society of Japan, No. 10 (1992), pp. 1,231-1,236). However, since such a cuprous halide is not dissolved in a silane compound which has heretofore been used as a medium material, such as tetraethoxysilane [Si(OCH.sub.2 CH.sub.3).sub.4 ], the solvent in which the cuprous halide is dissolved is limited. Further, the cuprous halide has a low solubility. Thus, the amount of the cuprous halide which can be uniformly dissolved in the sol is very low. As a result, the particulate material can be precipitated only in a low concentration even in a gel which is the reaction product. In general, the higher the concentration of the particulate material added is, the higher is the non-linear optical effect which can be expected. Thus, a process has been desired which comprises adding a cuprous halide in a high density to obtain a material having a high non-linear optical effect.
Under the foregoing circumstances, the inventors proposed a process for providing a non-linear optical element having a particulate metal or semiconductor dispersed therein in a high density which can be used as a crackless thin film having a sufficient thickness (JP-A-7-244305). In some detail, this process comprises mixing a solution of a matrix-forming substance having a functional group with a metal, semiconductor or precursor thereof to form a uniform solution, and then allowing the functional group to undergo reaction so that a matrix is formed while causing a particulate metal or semiconductor to be precipitated in the matrix.
However, this process has some disadvantages. For example, if a material which is subject to oxidation, such as cuprous halide, is precipitated, it is partly modified by oxidation, decomposition or the like during the heat precipitation process. The resulting product is colored yellow or brown due to absorptions other than absorption by excitons in the cuprous halide. Thus, the product leaves something to be desired in optical properties.